Question

Could someone give me a good, practical definition of what a data warehouse is?

Answer 1

I’m surprised no one has posted Inmon’s definition:

A warehouse is a subject-oriented,
integrated, time-variant and
non-volatile collection of data in
support of management’s decision
making process

From the same page you can pick up Kimball’s definition:

A copy of transaction data
specifically structured for query and
analysis

I think that, unfortunately, data warehousing is a wide-ranging field. There is a lot of variety with very few standard paradigms, specifically I’m thinking of Kimball’s dimensional modelling. Inmon does not have as a specific a methodology as Kimball’s and thus some 3NF models may or may not conform to his principles.

Because Inmon has broadened his scope for what warehousing is meant to accomplish, it can encompass unstructured data. However, analysis of unstructured data is very different than traditional analysis.

As applied to SQL Server, typically the largest Data Warehouses on SQL Server are modelled dimensionally, because this lends itself well to the non-distributed, non-massively parallel model. Massively parallel systems like Teradata generally perform a lot better with 3NF models. These are still table-based systems with the various tables connected with foreign key constraints (perhaps not enforced, but at least logical).

Of course, we are also seeing NoSQL data processing systems like Map/Reduce which are not really databases at all in the sense of normalized, denormalized or non/poorly-normalized relational databases which we have had for 40 years now.